Our crowded subsurface – Part II
I fell in love in 2008. As I recall, it was spring and unusually hot. The setting was a romantic island in Lake Mälaren, Sweden, with glimmering water, spring leaves and rolling meadows. The object of my love was Mr. John Deere and what he carried. Sweet memories.
First it was Birka
Mr. John Deere was a tractor, green of course, and his load was the first MIRA (MALÅ Imaging Radar Array) used for a large-scale archeological investigation. During two sunny field days, we (today Guideline Geo, then MALÅ Geoscience) covered 30 000 m2 ground and collected data points in an 8×8 cm grid. I learned to drive a tractor and dreamt of a future including green tractors, yellow boxes and applications limited to treasure hunts only.
The dream ended quite quickly in hard facts; measurements on the Viking age settlement Birka summed up to almost 5 million recorded traces, 375 km of GPR profile and huge amounts of gigabytes. The love suffered some blows, inside a dull office, trying to post-process all the data. Start, process, not enough memory space, computer crash. Start, process, not enough memory space, computer crash. Start, process, not enough memory space, computer crash. I bought a new computer and love was back. With amazing results.
Left: Me, my John Deere and the MIRA antenna box. Mid: The post holes seen on the inside of the city wall are only 25 cm in diameter. And yes, the narrow part indicates one of the city gates. Right: Deeper down, foundations of an house are seen. Notice the dot, a pit, maybe used for waste disposal inside the house.
Shortly after, I had the opportunity to resume my relationship with Mr. John Deere and the MIRA for investigations of a garden around a castle in south Sweden. Data collection, smooth and easy, but results were hard to understand. Round large circles in the grass areas? Why? For what? Huge pits? A circus arena? Just effects of measurement pattern or something real? I am not an archaeologist, merely a geophysicist, but one with a curiosity. After long discussions with the customer, he finally remembered an old painting, and voilà! Horse training tracks. Why didn´t I think of that?
The horse tracks, shown bottom right, created visible signs in MIRA data acquired 400 hundred years later.
These were my first experiences collecting, and processing, MIRA data. These were also the first large-scale multi-channel GPR archeological investigations ever made and, from here, the methodology has developed continuously, to give us a better and better insight of our crowded subsurface. So, today, in 2021 multichannel GPR investigations are a standard method for many of the archaeological investigations carried out around the world.
Then it was GPR for archaeology
Our crowded subsurface does not only contain utilities and other modern installations (read more here) but also, as said, the thrilling, exciting and important remains of our ancestors. These consist of not just fragile remains, including post-holes, hearths, or subtle wooden remains, but also more robust stone constructions such as walls and roads. Size varies from minor to huge, and from features that break the surface to those that are buried at considerable depth. The knowledge of our ancestors gives us both a historical understanding of the past but also insight for the future. I am happy to have an occupation where we can help in revealing this. Sometimes you need to do it, to know where to dig (either to target the archaeology for research, or to avoid the archaeology during a construction project), other times you just want a blueprint of the remains, to put on record, and then leave the ground as is, in peace, undisturbed.
As many of you may know single channel GPR has, for a long time, been one of the key tools for investigating archaeological remains. I did my first investigation around 1993-5 at the Royal Institute of Technology, Stockholm. Back then you worked slow, and I mean really slow. Place the antennas on a measurement tape. Press measure. Wait. Move antennas 50 cm. Press measure. Wait. Move antennas 50 cm. Press measure. Wait. Move antennas 50 cm. Results were printed ON PAPER in real-time, but if you were lucky, you could also do some post-processing. In DOS software if anybody remembers? You may give away your age, if you do!
From those early days, the single channel systems have become extremely fast, you can now collect data more or less continuously, with the possibility to set the in-line trace spacing as low as <1 cm. Work for archeological remains is normally done in dense parallel profile, around 25 cm, so data are good enough to be used in 3D for production of subsurface ‘blueprints’, so-called time-slices. And with the multi-channel options, the density of data points has increased even more, providing outstanding resolution. Now we are down to a profile spacing of 6.5 cm with the MALÅ MIRA HDR! And at highway speed! And user-friendly data collection and processing interfaces!
I am very proud of what our multi-channel systems have revealed around the world since the beginning of the MIRA era.
Or what do you say about the following few examples? Impressive, say I:
- The Roman city of Carnuntum, the Gladiator school, in Austria.
- Pre-Viking ships, wooden longboats, in Norway
- New constructions around Stonehenge, in Great Britain
- Viking Age and medieval settlements, in Denmark
- Iron Age settlements in Uppåkra, Sweden.
- Iron Age and Medieval Ringforts on Öland, in Sweden
- More discoveries on Viking Age settlements on Birka, in Sweden
I am also impressed by my colleagues. And to be honest, a bit jealous. Impressed by our development and production team creating these super systems, and jealous of today’s team members, who have been or are working with archaeological investigations.
My introductions to archaeological investigations were grand, with Birka, but then I left MALÅ Geoscience/Guideline Geo for ten years. My work as a consultant, at a geotechnical department, contained too few searches for underground archaeological features. And during my years off, I often rented and discussed equipment and archaeological investigations with Andreas Viberg, today one of our product managers for MALÅ, in his process of finalizing his PhD on archaeological geophysics in Sweden. I also observed Jimmy Adcock, today product manager for ABEM, on TV and Time Team in the UK, doing just fun investigations on a huge number of different archaeological sites. Yes, only a bit jealous.
So, today, when I am back where I started, I hope we can keep up the fantastic work for you to reveal what is hidden in our crowded subsurface, by creating durable, field-worthy equipment and smooth flow from data collection to report.
Jaana Gustafsson, Applications Specialist, Phd